The predominant protein of milk is casein, a phosphoprotein of high nutritional value, which is present in its native state in milk as a colloidal suspension of micellar agglomerates comprising .alpha.-, .beta.- and .kappa.-casein, plus phosphate and calcium. The casein protein(s), which comprise approximately 3% by weight of fluid cows' milk, may be distinguished from the so-called whey proteins by their insolubility at isoelectric pH 4.6, and casein is most commonly isolated by processes which involve acidification of skim milk to the isoelectric pH level. In commercial practice, pH reduction is achieved either by direct addition of an appropriate food-grade acid to skim milk or by formation of lactic acid in situ by bacterial fermentation of the lactose in the skim milk.
The casein coagulum (curd) produced by acidification of skim milk contains some non-protein milk solids such as mineral salts and lactose which are removed by washing the curd several times in water. The washed curd can then be dried directly to yield a product known as acid casein, or rendered soluble prior to drying by reaction with an alkali or alkaline salts. The soluble derivatives are known as caseinates and these products have physico-chemical, functional, and nutritional properties which make them useful ingredients in a wide variety of food preparations. Caseinates for example will act as emulsifying, thickening, water binding and foam stabilizing agents in foods. In some applications, such as cheese analogs, the caseinates not only function as emulsifiers of oil in water, but also exhibit thermoplasticity which promotes authentic cheese melt characteristics in the cheese-like substance. In this latter property casein and caseinates are somewhat unique among those commercially-available protein isolates and concentrates which are suitable for use in formulated food products.
There are potentially available abundant quantities of vegetable protein, to be derived from legumes, cereal grains, and nuts. A number of soy protein isolates and concentrates are already available commercially and these proteins exhibit some of the functional properties of casein and caseinates. However, classic isolated soy proteins are not thermoplastic, which limits the use of these products in certain food applications as described above. Wheat gluten, on the other hand, is a protein concentrate which does exhibit thermoelastic properties and it could possibly be used together with, or in place of, casein in formulated food systems, were it not for the extreme insolubility of the gluten proteins.
It is generally known that wheat gluten proteins can be rendered soluble by acid hydrolysis as described by Holme and Briggs in Cereal Chemistry, Volume 36, Page 321 (1959). However, published procedures for the preparation of acid-solubilized wheat gluten, as described by Wu, Nakai and Powrie in the Journal of Agriculture and Food Chemistry, Volume 24, Page 504 (1976), suggest the need for high heat treatment of an aqueous gluten slurry in an autoclave at a low pH. Such a solubilization process is considered to be impractical for implementation on a large production plant scale, firstly because of equipment requirements and secondly because excessive quantities of acid are needed. However, if the acidified wheat gluten solution were to be used as the precipitant in the manufacture of acid casein from skim milk, the practical disadvantage of excessive acid usage would be greatly diminished, since an approximately equivalent amount of acid would be required, in any event, to acidify the skim milk to the isoelectric pH of casein. It was believed that if high temperature high pressure treatment of wheat gluten could be avoided by control of reaction time, it would be possible to produce, in an acid solution, non-devitalized soluble wheat proteins which will precipitate at the same pH as casein in combination with skim milk.